1. Field of the Invention
The present invention relates to a light emitting device and a method of fabricating the same, and more particularly, to a light emitting device having a plurality of light is emitting cells having a vertical structure in which an n-electrode and a p-electrode are respectively formed on upper and lower portions of each light emitting cell.
2. Discussion of the Background
GaN-based light emitting diodes (LEDs) are widely used for display and backlights. Further, LEDs have less electric power consumption and a longer lifespan as compared with conventional light bulbs or fluorescent lamps, so that the LEDs have been substituted for conventional incandescent bulbs and fluorescent lamps and their application areas have been expanded to the use thereof for general illumination.
Recently, LEDs have been commercialized, which are directly connected to a high-voltage DC or AC power source to emit light. For example, an LED capable of being directly connected to a high-voltage DC or AC power source is disclosed in PCT Patent Publication No. WO 2004/023568A1 (SAKAI et. al.), entitled “LIGHT-EMITTING DEVICE HAVING LIGHT-EMITTING ELEMENTS.”
According to PCT Patent Publication No. WO 2004/023568A1, LEDs are two-dimensionally connected on a single insulative substrate such as a sapphire substrate to form serial LED arrays. Such serial LED arrays can be driven by a high-voltage DC power source. Further, there is provided a single-chip light emitting device capable of being driven by a high-voltage AC power source by allowing such LED arrays to be connected in reverse parallel on the sapphire substrate.
Since the light emitting device has light emitting cells formed on a substrate used as a growth substrate, e.g., a sapphire substrate, the light emitting cells have a limitation in structure, and there is a limitation in improving light extraction efficiency. To solve such a problem, a method of fabricating a light emitting device having a plurality of light emitting cells using a substrate separation process is disclosed in Korean Patent No. 10-0599012, entitled “LIGHT EMITTING DIODE HAVING THERMAL CONDUCTIVE SUBSTRATE AND METHOD OF FABRICATING THE SAME.”
FIGS. 1 to 4 are sectional views illustrating a method of fabricating a light emitting device according to a prior art.
Referring to FIG. 1, semiconductor layers comprising a buffer layer 23, an n-type semiconductor layer 25, an active layer 27 and a p-type semiconductor layer 29 are formed on a sacrificial substrate 21. A first metal layer 31 is formed on the semiconductor layers, and a second metal layer 53 is formed on a substrate 51 that is separate from the sacrificial substrate 21. The first metal layer 31 may comprise a reflective metal layer. The second metal layer 53 is joined with the first metal layer 31 so that the substrate 51 is bonded on the semiconductor layers.
Referring to FIG. 2, after the substrate 51 is bonded, the sacrificial layer 21 is separated by a laser lift-off process. Also, after the substrate 21 is separated, the remaining is buffer layer 23 is removed, and a surface of the n-type semiconductor layer 25 is exposed.
Referring to FIG. 3, the semiconductor layers 25, 27 and 29 and the metal layers 31 and 53 are patterned using photolithography and etching techniques to form metal patterns 40 spaced apart from one another and light emitting cells 30 positioned on regions of the respective metal patterns 40. Each of the light emitting cells 30 comprises a patterned p-type semiconductor layer 29a, a patterned active layer 27a and a patterned n-type semiconductor layer 25a. 
Referring to FIG. 4, metal wires 57 are formed to electrically connect top surfaces of the light emitting cells 30 to the metal patterns 40 adjacent thereto. The metal wires 57 allow the light emitting cells 30 to be connected therethrough, thereby forming a serial array of light emitting cells. Electrode pads 55 for connecting the metal wires 57 may be formed on the n-type semiconductor layers 25a. Electrode pads may also be formed on the metal patterns 40. Two or more arrays may be formed and these arrays are connected in reverse parallel, so to that an LED capable of being driven by an AC power source is provided.
According to the prior art, thermal dissipation performance of the LED can be improved since the substrate 51 can be selected from a variety of substrates, and light extraction efficiency can be enhanced by treating a surface of the n-type semiconductor layer 25a. Further, a first metal layer 31a comprises a reflective metal layer and reflects light traveling from the light is emitting cells 30 toward the substrate 51, so that the light emitting efficiency can be more improved.
However, in the prior art, while the semiconductor layers 25, 27 and 29 and the metal layers 31 and 53 are patterned, etch byproducts of a metallic material may be stuck to side walls of the light emitting cells 30, and therefore, a short circuit between the n-type semiconductor layer 25a and the p-type semiconductor layer 29a may occur. Further, a surface of the first metal layer 31a, which is exposed while the semiconductor layers 25, 27 and 29 are etched, may be easily damaged by plasma. When the first metal layer 31a comprises a reflective metal layer such as Ag or Al, such etching damage may be serious. Since the surface of the metal layer 31a is damaged by plasma, the adhesion of the wires 57 or electrode pads formed on the metal layer 31a is lowered, resulting in a device failure.
Meanwhile, according to the prior art, the first metal layer 31 may comprise a reflective metal layer, thereby reflecting light traveling from the light emitting cells 30 toward the substrate 51. However, etching damage may occur on the reflective metal layers that are exposed to a space between the light emitting cells 30, and the reflective metal layers may be easily oxidized due to their exposure to the outside. Particularly, the oxidation of the exposed reflective metal layers is not limited to the exposed portions but progresses toward regions below the light emitting cells 30, thereby lowering reflectivity of the reflective metal layers.
Further, in the prior art, since the metal pattern is in contact with the bottom surface of each light emitting cell, current in a lower semiconductor layer can be smoothly distributed through the metal pattern. However, the position of the metal wire that is in contact with the top surface of each light emitting cell is limited to a corner or edge of the light emitting cell. Therefore, current is necessarily distributed through an upper semiconductor layer. However, since the semiconductor layer 25a generally has a relatively higher specific resistivity than a metal material layer, current is not smoothly distributed but concentrated on the contact portion of the metal wire. The concentration of the current degrades the light emitting efficiency of the light emitting cells.